Kinetics of Phosphorus Release from Vivianite, Hydroxyapatite, and Bone Char Influenced by Organic and Inorganic Compounds

Rates and Mechanism of Vivianite Dissolution under Anoxic Conditions

The iron phosphate mineral vivianite Fe(II)3(PO4)2·8H2O has emerged as a potential renewable P source. Although the importance of vivianite as a potential P sink in the global P cycle had previously been recognized, a mechanistic understanding of vivianite dissolution at the molecular level, critical to its potential application, is still elusive. The potential of vivianite as a P sink or source in natural or engineered systems is directly dependent on its dissolution kinetics under environmentally relevant conditions. To understand the thermodynamic and kinetic controls on bioavailability, the oxidation and dissolution processes of vivianite must be disentangled. In this study, we conducted controlled batch and flow-through experiments to quantitatively determine the dissolution rates and mechanisms of vivianite under anoxic conditions as a function of pH and temperature. Our results demonstrate that vivianite solubility and dissolution rates strongly decreased with increasing solution pH. Dissolution was nonstoichiometric at alkaline pH (>7). The rapid initial dissolution rate of vivianite is related to the solution saturation state, indicating a thermodynamic rather than a kinetic control. A defect-driven dissolution mechanism is proposed. Dissolution kinetics over pH 5-9 could be described with a rate law with a single rate constant and a reaction order of 0.61 with respect to {H+}: R exp = 36.0 · e - 1.41 · pH · [ 1 - e ( 0.2 · Δ G / RT ) ] 4.7 The activation energy of vivianite dissolution proved low (Ea = 20.3 kJ mol-1), suggesting hydrogen bridge dissociation as the rate-determining step.

Dissolution Performance of Carbon/Hydroxyapatite Nanocomposite Prepared from Fish Scales

The utilization of waste as raw materials in the production of functional materials is a growing area of interest to promote global sustainable growth. This work studied the microstructure, chemical properties, and phosphorus (P) dissolution efficiency of carbon/hydroxyapatite (HAP) nanocomposites derived from fish scale wastes, namely FSBCs. Tilapia scales were pyrolyzed at different temperatures (450–600 °C) and the dissolution performance of the biochars in acidic media was evaluated. The pyrolysis of fish scales yielded biochars which are carbon/HAP nanocomposites of which the characteristics, including the specific surface area, carbon content, degree of crystallinity of HAP, and the degree of CO32− substitution in HAP crystals, were dependent on the pyrolysis temperature and had an influence on the dissolution efficiency. P dissolution experiments suggested that the specific surface area and the presence of carbon in the composite played an important role during the dissolution process. The presence of the organic residues on the carbon could potentially block the calcium sites on HAP from citric acid and lower the dissolution efficiency. The ratio of CO32−/PO43− substitution in HAP also affected the dissolubility of FSBCs. The manipulation of their surface area and the CO32−/PO43− ratio of these materials would allow us to increase its solubility, which could benefit a prospective application such as P fertilizer production.

Seedling Growth and Quality of Avicennia marina (Forssk.) Vierh. under Growth Media Composition and Controlled Salinity in an Ex Situ Nursery

Avicennia marina (Forssk.) Vierh. is an important mangrove species that inhabits the outermost zone of mangrove forests, but it has been shown to have a poor ability to regenerate due to its low seedling quality. We conducted a study to evaluate the specific growth requirements of A. marina, i.e., medium and salinity level. Germinated seeds were transplanted to pots filled with media, i.e., silt loam (M1), loam (M2), sandy loam (M3), or sand (M4), with various salinity levels 5 (S1), 5–15 (S2), 15–25 (S3), or 25–35 ppt (S4). Survival rate, growth, biomass partition, and seedling quality were observed for 14 weeks after transplanting the seeds. The highest rate of seedling survival was found in the S2 condition, and higher concentrations of salinity lowered the survival rates. The S1 treatment promoted the initial 8 week growth of the seedlings. Growth medium had no significant effect, except on the survival rates grown in M4. Growth medium composition had no distinct effect on seedling growth. The S2 and S3 treatments induced better growth (in terms of shoot height and root length) and resulted in high-quality (i.e., Dickson quality index) seedlings in any type of medium. The S3 treatment increased the seedling quality in M1 and M4, whereas the S4 treatment only benefited seedlings in the M4 medium. According to the results, a specific range of salinity (5–15 ppt) with circulated water in any type of medium is recommended for the establishment of an ex situ nursery for the propagation of A. marina, in contrast to the general range of salinity (4–35 ppt) stated in previous references.

Microbial Volatile Organic Compound (VOC)-Driven Dissolution and Surface Modification of Phosphorus-Containing Soil Minerals for Plant Nutrition: An Indirect Route for VOC-Based Plant-Microbe Communications

We investigated the ability of microbial volatile organic compounds (MVOCs) emitted by Bacillus megaterium (a well-known MVOC producer) to modify the dissolution kinetics and surface of hydroxyapatite, a natural soil mineral. Facilitated phosphate release was induced by the airborne MVOCs in a time-dependent manner. Use of each standard chemical of the MVOCs then revealed that acetic and oxalic acids are crucial for the phenomenon. In addition, the ability of such MVOCs to engineer the apatite surfaces was evidenced by FT-IR spectra showing the COO^- band variation with incubation time and the prolonged acceleration of phosphate release during the negligible acidification of the hydroxyapatite-containing solutions. The formation of calcium oxalate was revealed through SEM-EDS and XRD analyses, suggesting that MVOC oxalic acid interacts with calcium ions, leading to the precipitation of calcium oxalate, thus preventing the recrystallization of calcium phosphates. Gel- and soil-based plant cultivation tests employing Arabidopsis thaliana and solid calcium phosphates (i.e., nano- and microsized hydroxyapatites and calcium phosphate dibasic) demonstrated that these MVOC mechanisms facilitate plant growth by ensuring the prolonged supply of plant-available phosphate. The relationship between the growth enhancement and the particle size of the calcium phosphates also substantiated the MVOC sorption onto soil minerals related to plant growth. Given that most previous studies have assumed that MVOCs are a molecular lexicon directly detected by the dedicated sensing machinery of plants, our approach provides a new mechanistic view of the presence of abiotic mediators in the interaction between plants and microbes via MVOCs.

Single and Binary Fe- and Al-hydroxides Affect Potential Phosphorus Mobilization and Transfer from Pools of Different Availability

Phosphorus (P) fixation is a global problem for soil fertility and negatively impacts agricultural productivity. This study characterizes P desorption of already fixed P by using KCl, KNO3, histidine, and malic acid as inorganic and organic compounds, which are quite common in soil. Goethite, gibbsite, and ferrihydrite, as well as hydroxide mixtures with varying Fe- and Al-ratio were selected as model substances of crystalline and amorphous Fe- and Al-hydroxides. Especially two- and multi-component hydroxide systems are common in soils, but they have barely been included in desorption studies. Goethite showed the highest desorption in the range from 70.4 to 81.0%, followed by gibbsite with values in the range from 50.7 to 42.6%. Ferrihydrite had distinctive lower desorption in the range from 11.8 to 1.9%. Within the group of the amorphous Fe-Al-hydroxide mixtures, P desorption was lowest at the balanced mixture ratio for 1 Fe: 1 Al, increased either with increasing Fe or Al amount. Precipitation and steric effects were concluded to be important influencing factors. More P was released by crystalline Fe-hydroxides, and Al-hydroxides of varying crystallinity, but desorption using histidine and malic acid did not substantially influence P desorption compared to inorganic constituents.